2,3,5-trimethylhydroquinone (TMHQ) is an important intermediate that is used in the production of vitamin E and vitamin E acetate. In addition to the known preparation processes from aromatic starting materials, 2,3,5-trimethylhydroquinone can be prepared from a non-aromatic compound, namely 2,6,6-trimethylcyclohex-2-ene-1,4-dione, by rearrangement under acylating conditions followed by hydrolysis.
Patent Specification DE 26 46 172 C2 describes a process in which 2,6,6-trimethylcyclo-hex-2-ene-1,4-dione is directly rearranged in the vapor phase at elevated temperature in contact with an acid catalyst to form trimethyl-hydroquinone. The yield in this process is however low (50% at 30% conversion). If the aromatization of 2,6,6-trimethylcyclohex-2-ene-1,4-dione is carried out in the presence of an acylating agent, then trimethylhydroquinone diesters are obtained that yield trimethylhydroquinone on subsequent hydrolysis.
According to Bull. Korean Chem. Soc. 1991, 12, 253, for example, the rearrangement is carried out in 5% solution acetic anydride by adding five equivalents of concentrated sulfuric acid. The trimethylhydroquinone diester is in this case obtained only in a 30% yield.
In a further process according to DE-OS 2 149 159, 2,6,6-trimethylcyclohex-2-ene-1,4-dione can be converted in the presence of acetic anhydride in a proton-catalyzed or Lewis acid-catalyzed rearrangement to form trimethylhydroquinone diacetate, which is then saponified to trimethyl-hydroquinone. Moderate to good yields are said to be obtained in this process.
From DE-OS 196 27977, it is known to prepare TMHQ by reacting ketoisophorone with an acylating agent in the presence of very strong acids, for example fluorosulfonic acid or oleum. In this process the initially formed corresponding ester is then saponified.
The known processes have the disadvantage that either the yields are low or that corrosion problems arise at the same time due to the use of strong dissolved acids, which make it necessary to use high quality materials. The separation and recycling of the dissolved catalyst are also difficult to carry out.